1. Field of the Invention
The present invention relates to the field of well logging. More specifically, the present invention relates to apparatus and methods for well logging using neutron tools.
2. Background Art
In traditional wireline (WL) well logging, a sonde is lowered into a borehole and data are collected as the tool is pulled uphole. The detectors in the sonde therefore make measurements as a function of depth or acquisition time, and the precise location of the detectors with respect to the circumference of the borehole at each depth or acquisition time, i.e., the azimuthal position, is generally disregarded. Measurements are typically recorded as a function of acquisition time (temporal intervals) and are later converted to measurements as a function of depth of the well (axial depth; spatial intervals). In this description, “acquisition intervals” are generally used to refer to either spatial or temporal intervals, i.e., the “depth” (or “axial depth”) of a well or the “acquisition time.” Notable exceptions include formation micro-scanners such as those disclosed in U.S. Pat. No. 4,468,623 issued to Gianzero et al. One example of a formation micro-scanner is that under the trade name of formation micro-imager (FMI™) from Schlumberger Technology Corp. (Houston, Tex.). The sonde of a formation micro-scanner forces several articulated pads against the borehole wall to measure the formation resistivity as a function of depth and azimuth. The resulting data may be used to construct a formation image. They provide detailed and valuable information about the formations that are otherwise unavailable, such as the direction and inclination of dipping beds.
With measurement-while-drilling (MWD) or logging-while-drilling (LWD), a tool usually rotates with the drill string, and, therefore, the detectors in the tool sweep through the circumference of the borehole many times at each depth level. One example of an LWD nuclear measurement tool is that under the trade name of ADN™ from Schiumberger Technology Corp. (Houston, Tex.). This tool can record gamma density and neutron data as a function of tool orientation. If the measurements are well-focused, then the measurement data can be used to build an image of the borehole. In the field of nuclear measurements, gamma ray data are more focused. Therefore, azimuthal nuclear logging traditionally involves gamma density measurements. In contrast, neutron measurements are usually considered unfocused and have not been used for formation imaging.
The use of azimuthal sectors to improve gamma density data analysis is disclosed in U.S. Pat. No. 5,091,644 issued to Minette. This patent describes a method for analyzing gamma density data from an MWD logging tool. According to this method, the received gamma ray signals are binned into a plurality of sectors, typically four sectors: bottom, top, left, and right. The binning divides the data into four azimuthal sector data for each detector. The relative errors of the four azimuthal sector data are compared. The method then selects a specific azimuthal data that has the least error as an optimum measurement. Alternatively, the method combines measurements from two or more sectors to produce an optimum measurement.
U.S. Pat. No. 5,397,893 issued to Minette describes a similar method that can be used to separate gamma density data into a number of bins (sectors) based on the amount of standoff. Then, these density measurements are combined in a manner that minimizes the total error in the density calculation.
U.S. Pat. No. 5,473,158 issued to Holenka et al. discloses methods and apparatus for measuring formation characteristics as a function of azimuthal angles using neutron, gamma, and ultrasonic devices. This patent is assigned to the same assignee as the present invention and is hereby incorporated by reference. In a method according to this patent, neutron data are measured as a function of quadrants in a conventional manner. The bottom point where the tool presumably contacts the borehole wall is then identified using the angular position sensors provided in the tool. The measurements in the bottom quadrant are presumed to have relatively less error and are more representative of the formation properties. Thus, the methods of Holenka may be used to select data from the “bottom” sectors as representative of formation properties. However, these methods cannot provide formation images from the neutron measurements.
Because formation images may provide information that is otherwise unavailable from conventional neutron measurements, it is desirable to have methods and apparatus that permit azimuthal neutron measurements that can be used to provide formation images.